1. Field of the Invention
The present invention relates to radial tires and tire reinforcing steel cords used for the radial tires, and more specifically, the invention relates to radial tires for use in trucks and buses and tire reinforcing steel cords used therefor.
2. Related Art Statement
As the radial tires for use in the conventional trucks and buses, there have been used a radial tire body-reinforced by a belt composed of at least three rubberized cloth layers in which the cord cloth layers containing steel cords obliquely arranged at a relatively small angle with respect to the tire equatorial plane are so piled that the cords of the adjacent layers may be intersected with one another and a carcass composed of at least one rubberized cord cloth having cords arranged in substantially perpendicularly intersected with respect to the equatorial plane.
The heavy duty tires for use in trucks and buses employing steel cords as reinforcing elements for the belt and the carcass are used not only on fully-paved and well-conditioned good roads for instance a route for the exclusive use of motor vehicles such as expressway or the like but also on running roads partly including bad roads with an inferior road surface such as a construction road or the like.
In the former case, interlayer separation (BLB) of the belt layers is likely to occur due to running at high speeds while in the latter case, bursting, chipping and separation between the tread layer and a breaker layer, (TLB), are often developed from cuts at a time from running intermediate period to the running terminal period so that there are likely to cause problems with respect to abnormal shortening in the use life, impossibility of recapping and safety.
Tires aiming at preventing the problems in the latter case as much as possible are likely to produce the BLB which affects the performance required on the other hand on the good roads to cause the burst.
Further, when the conventional cords used in the tire of this kind are used in the belt layers in a form of a three layer twisted construction or two layer twisted construction containing steel filaments of less than 0.32 mm in filament diameter, the Young's modulus of the cords has been tried to be lowered by making the twisting pitch smaller so as to enhance the bad road performance. However, it has been proved that the durability performance (BLB resistant performance) at high speed running is lowered. Incidentally, in the case of the tire structure that the belt layer adjoining to the carcass ply among the belt layers is made hollow at the central portion thereof from the standpoint of the tire structure as having conventionally employed to enhance the bad road performance, it has been impossible to well overcome the defects of the radial tires for use on good roads containing at least partially bad roads, and has a problem that the durability is lowered.
Due to their large Young's modulus, the conventional steel cords have excellent good road performance but are poor in enveloping property (Ep). Thus, cuts are likely to be produced when treading projections such as gravels or pebbles. In order to eliminate such a defect, there has been examined a center hollow structure. In the case of the center hollow structure, the Ep property is improved at the central portion of the tire tread, whereas because the center hollow belt layer is present in the tire shoulder portion, the Ep property can not be improved and improving effect against cracks at belt edges on bad roads is small. Further, heat generation is large on the good road running due to small belt rigidity so that the BLB is likely to take place.
When the Young's modulus of the cords are simply lowered, there occurs the problem that while the bad road performance is enhanced, the durability in the good road performance is deteriorated due to its lowered belt rigidity as in the case of the center hollow structure [while the durability is not different in the case of bad roads with a bad road percentage of 40% due to the slow running speed, cracks are produced twice in the case of the good road (with a bad road percentage of 5%) including bad roads to deteriorate the durability. This is considered that since the speed factor becomes important, the heat generation increases and the crackproducing factor sensitivity depends upon the physical properties of cords.] The durability referred to here means the performance evaluated based on the cracks at the edge portion of the belt.